Method for actuating a wheel brake device

ABSTRACT

The invention relates to a method for actuating a wheel brake assembly, in particular an electromechanical wheel brake assembly ( 10 ) or a mechanical system involving friction and having spring elasticity. To increase a braking force once a quasi-static terminal state of the wheel brake assembly ( 10 ) is reached, the invention proposes actuating the wheel brake assembly ( 10 ) for a brief period of time in the release direction and then to re-tighten it; the period of time of the actuation in the release direction is selected to be so brief that the braking force is reduced, if at all, only imperceptibly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 00/02056 filed onJun. 23, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for actuating a wheel brake assembly,in particular an electromechanical wheel brake assembly or a mechanicalsystem involving friction and having spring elasticity.

2. Description of the Prior Art

For the sake of clear illustration, the invention will be explainedbelow concretely in terms of the actuation of an electromechanical wheelbrake assembly. However, the invention is not intended to be limited toelectromechanical wheel brake assemblies; for instance, the method ofthe invention is applicable to hydraulic or pneumatic wheel brakeassemblies, especially if they are actuated by external force, andgenerally to mechanical systems involving friction and having springelasticity.

Electromechanical wheel brake assemblies are known per se. For example,see German Patent Disclosure DE 42 29 042 A1. The knownelectromechanical wheel brake assemblies have an electric motor, whoserotary motion is transmitted by a gear to a rotation/translationconversion gear, typically a spindle drive, with which a friction brakelining can be pressed (tightening) against a brake body, connected in amanner fixed against relative rotation to a vehicle wheel, such as abrake disk or a brake drum, and lifted again (releasing). To convert therotary motion of the electric motor into a translational motion forexerting pressure on the brake lining, a cam can for instance also beused. It is also known to embody a rotor of the electric motor as a nutof the spindle drive and to dispense with the gear between the electricmotor and the spindle drive.

For braking, in the known electromechanical wheel brake assemblies theelectric motor is supplied with current in the tightening direction,until a desired a braking force is reached. The braking force can beincreased until such time as a quasi-static terminal state is attained,at which a torque of the electric motor, at maximum current supply, nolonger suffices to increase the contact pressure of the friction brakelining against the brake body any further.

SUMMARY OF THE INVENTION

The method of the invention has the advantage that the braking force ofthe wheel brake assembly can be increased beyond the value that it hasin the quasi-static terminal state, and the braking action is improvedsubstantially.

Another advantage of the method of the invention is that it requires noexternal measurement or input signals, such as the braking force exertedby the wheel brake assembly, in order to be performed. Sensors thatmeasure the contact pressure of the friction brake lining at the brakebody, or perform similar actions, can therefore be omitted. The methodof the invention can be employed in an existing and in particularelectromechanical wheel brake assembly, without having to make suchmodifications as mounting a sensor on the wheel brake assembly.

In fading as well, that is, when the braking force of the wheel brakeassembly fades as a consequence of overheating, the method of theinvention can be employed to advantage to regain or even exceed thebraking force that existed before the fading occurred.

The invention is based on the following concept: The wheel brakeassembly is not absolutely rigid; even when embodied stiffly, it hassome elasticity, against which the electric motor tightens the wheelbrake assembly. The electric motor upon tightening must also overcomefriction, for instance of the gear and the spindle drive, and because ofthe increasing forces, this friction load increases as the tighteningincreases. At a high tightening force, the friction load is high; thatis, a not insignificant proportion of the torque of the electric motoris consumed to overcome the friction, and only the torque of theelectric motor beyond that proportion increases the tightening forcefurther. When the quasi-static terminal state is reached, the movingparts of the wheel brake assembly come to a stop, and the frictionchanges into static friction, which is higher. A further increase in thebraking force would be possible then only if the torque of the electricmotor could be increased so far that the static friction is overcome,and that the moving parts of the wheel brake assembly move again.

Since the torque of the electric motor cannot be increased arbitrarily,in the method of the invention a different course has been taken: Toincrease the braking force further once the quasi-static terminal statehas been reached, the wheel brake assembly is actuated for a briefperiod of time in the release direction and then re-tightened. By theactuation in the release direction, and specifically with reinforcementfrom the elasticity of the wheel brake assembly rather than counter tothe elasticity, the static friction is overcome, and the moving parts ofthe wheel brake assembly are put into motion again. After that, thewheel brake assembly is tightened again, and the braking force isgreater than in the quasi-static terminal state, since the staticfriction need not be overcome; instead, the electric motor operatescounter to the lesser sliding friction. The method can be repeatedmultiple times, in order to increase the braking force further inincrements. In experiments, it was possible to increase the brakingforce by approximately one-third compared to the value in thequasi-static terminal state.

For actuating the wheel brake assembly in the release direction, theelectric motor need not necessarily be supplied with current in therelease direction; often, it suffices to interrupt its current supply orreduce it, before the electric motor is again acted upon with maximumcurrent supply in the tightening direction in order to re-tighten thewheel brake assembly. Nor is the wheel brake assembly actually released;instead, the actuation in the release direction is so brief that thebraking force is reduced, if at all, only imperceptibly. It is not thegoal of the invention to reduce the braking force of the wheel brakeassembly temporarily and then increase it again; instead, by actuatingthe wheel brake assembly in the release direction, any stresses inbearings, gears, guides and the like, which can occur in thequasi-static terminal state because of the high tightening force of thewheel brake assembly, are meant to be reversed, and the static frictionis to be overcome. An explanation for why the braking force of the wheelbrake assembly does not decrease despite a brief actuation in therelease direction could be hysteresis resulting from the elasticity ofthe wheel brake assembly. In any case, in experiments, no loss ofbraking force during the brief actuation of the wheel brake assembly inthe release direction was measurable. This can be due to the fact eitherthat the braking force in fact did not decrease, or that the decrease inbraking force was less than the measurement precision and hence wasinsignificant. A perceptible reduction in the braking force during theactuation of the wheel brake assembly in the release direction would bequite worrisome to a driver and would moreover lengthen the brakingdistance, which should be avoided and is unwanted according to theinvention. What is meant by the expression that the braking force isreduced if at all only imperceptibly is that the wheel brake assembly isactuated in the release direction only so briefly that any stresses inthe drive of the wheel brake assembly will be reversed and the staticfriction will change into a sliding friction.

The method of the invention can also be adopted for other mechanicalsystems involving friction and having spring elasticity and is notlimited to wheel brake assemblies. The above explanations on the wheelbrake assembly, in particular on the electromechanical wheel brakeassembly, logically apply here as well and will not be repeated at thispoint.

For increasing the braking force incrementally, the method according tothe invention is repeated multiple times. The method is repeated after apredetermined time after the onset of the re-tightening. This has theadvantage that there is no need to determine or wait for whether thewheel brake assembly, after being re-tightened, has already come to astop. The method is repeated if the wheel brake assembly/the system uponre-tightening has come to a stop or at least is virtually at a stop. Inthis way, each time the method is repeated a maximum increase in thebraking force, or in the force exerted by the spring-elastic mechanicalsystem that involves friction is attained.

Since the braking force, or the force exerted by the system, cannot beincreased arbitrarily by the method of the invention but instead seeksto meet a limit value, the number of repetitions is limited.

According to one aspect of the invention, a distance that an actuatingelement of the wheel brake assembly covers in its actuation in therelease direction is used as a standard for the actuation in the releasedirection. If the actuating element has traveled a fixed distance in therelease direction, then the wheel brake assembly/the system isre-tightened. The travel of the actuating element in the releasedirection accordingly determines the brief period of time during whichthe wheel brake assembly/the system is actuated in the releasedirection. In this case, no time is measured. The actuating element canfor instance be a rotor of the electric motor of the wheel brakeassembly, which is rotated backward by a predetermined angle in order toactuate the wheel brake assembly in the release direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below in terms of anexemplary embodiment shown in the drawings, in which:

FIG. 1 is an overview of an electromechanical wheel brake assembly; and

FIG. 2 is a timing graph to illustrate the mode of operation of themethod of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an overview of an electromechanical wheel brake assembly10, known per se. Reference numeral 12 indicates an electronic controlunit, which via an output line 14 controls an electric motor 16. Theelectric motor 16 is part of a brake actuator 18, which via a mechanicalconnection 20 actuates a brake device 22, with which a brake body 24,such as a brake disk, which is connected to a vehicle wheel, not shown,in a manner fixed against relative rotation can be braked. As its inputsignal, the control unit 12 receives a signal of a brake pedal sensor26, which measures a force by which a brake pedal is depressed, or atravel distance by which the brake pedal is depressed.

In an experimental setup, not shown, for performing the method of theinvention, the wheel brake assembly 10 had a plate wheel gear, which wasdrivable with the electric motor 16 and whose plate wheel was mounted ina manner fixed against relative rotation on a nut of a spindle drive.The plate wheel gear and the spindle drive form the brake actuator 18.With a spindle of the spindle drive, a friction brake lining located ina brake caliper could be pressed against a brake disk 24 of the wheelbrake assembly, embodied as a disk brake assembly. The spindle forms themechanical connection 20, while the brake caliper forms the brake device22. Because of the elasticity, especially of the brake caliper but alsoof the other components of the wheel brake assembly, that exist even ina rigid design, the mechanical connection 20 can be thought of as aspring element.

Upon the usual actuation of the wheel brake assembly 10, as a functionof what the driver wants, the electric motor 16 is supplied with currentin a tightening device until such time as a braking force dependent onwhat the driver wants is reached. The maximum attainable braking forceoccurs when the electric motor 16 is subjected to a maximum currentsupply. It tightens the wheel brake assembly 10 until its torque nolonger suffices to increase the tightening force further. This is theso-called quasi-static terminal state.

If the braking force is to be increased further, then according to theinvention the wheel brake assembly 10 is actuated for a brief period oftime in the release direction; for this purpose, it can suffice tointerrupt the current supply to the electric motor 16 briefly, and thereis no absolute necessity of supplying current to the electric motor 16in the release direction. Next, the electric motor 16 is again suppliedwith maximum current in the tightening direction. By the brief actuationof the wheel brake assembly 10 in the release direction, any stressesexisting in the brake actuator 18 because of the high tightening forcehave gone away, and a static friction, which ensues because of thestoppage of the wheel brake assembly 10 once the quasi-static terminalstate has been reached, has been overcome, specifically by utilizing theelasticity of the wheel brake assembly 10. In the ensuing re-tightening,an increased braking force is thereby attained, which is beyond that ofthe quasi-static terminal state. The period of time for actuating thewheel brake assembly 10 in the release direction is selected to be sobrief that the braking force is either not reduced, or at most isreduced imperceptibly.

For increasing the braking force further in increments, the method ofthe invention is repeated. The repetition can be done when the electricmotor 16 and the brake actuator 18, upon re-tightening, are completelyat a stop or nearly at a stop. The repetition of the method can also bedone after a fixedly defined period of time, which can end even beforethe stoppage of the electric motor 16 and the brake actuator 18 uponre-tightening.

Even if the braking force of the wheel brake assembly 10 fades, forinstance because of overheating (fading) during braking, by applicationof the method of the invention the braking force can be restored to itsoriginal value or even increased beyond it. The use of the method of theinvention is not limited to electromechanical wheel brake assemblies; inparticular, it can also be used for electrohydraulic or electropneumaticbrake systems. The method of the invention is furthermore not limited towheel brake assemblies; it can also be adopted in other mechanicalsystems involving friction and having spring elasticity.

A preferred realization of the method of the invention is for it to beimplemented as a program in a microcomputer of the control unit 12.

The course of the method of the invention can be seen from the timinggraph in FIG. 2. Here the course of the contact pressure F of a frictionbrake lining against the brake disk 24 is represented by a dot-dashedline, and a rotational angle w of a rotor of the electric motor 16 isplotted with a dashed line, both over the time t. The electromechanicalwheel brake assembly 10 used in the experimentation has been describedabove at the beginning of the description of the exemplary embodiment.F₀ indicates the contact pressure of the friction brake lining againstthe brake disk at maximum current supplied to the electric motor 16,that is, in the quasi-static terminal state of the wheel brake assembly10. To increase the contact pressure F further, in the experimentdescribed the rotor of the electric motor 16 is rotated backward at timeT₁ by an angle of approximately 15° in the release direction, and thenthe electric motor 16 was supplied with current again in the tighteningdirection. As shown in FIG. 2, upon re-tightening, the rotor of theelectric motor 16 rotates past its position in the quasi-static terminalstate; the contact pressure F also increases in stages to a value F₁,which is greater than the contact pressure F₀ in the quasi-staticterminal state. At time T₂, the rotor of the electric motor 16 has beenrotated in reverse by 15° again, and then the electric motor has beensubjected to maximum current supply once again, as a result of which therotor has rotated onward again and the contact pressure F has beenincreased further to the value F₂. By repetition of the method, thecontact pressure F can be increased by approximately one-third, comparedto the contact pressure F₀ in the quasi-static terminal state of thewheel brake assembly 10. The period of time of the reverse rotation ofthe rotor by 15° and of the ensuing forward rotation past the originalposition was approximately 15 ms, and the chronological spacing betweentwo repetitions was approximately 0.15 to 0.2 seconds.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

1. A method for actuating a wheel brake assembly comprising the steps of(a) initially actuating the brake assembly in a tightening direction tocause a brake lining to be pressed against a brake body to establish aquasi-static terminal braking state, then (b) actuating the wheel brakeassembly (10) for a brief period of time in a release direction oppositeto the tightening direction, and then (c) again actuating the brakeassembly in the tightening direction, said brief period of time of theactuation in the release direction being selected to be so short thatany reduction of the braking force is imperceptible.
 2. The method ofclaim 1 further comprising repeating steps (b) and (c).
 3. The method ofclaim 2, wherein steps (b) and (c) are repeated after a predeterminedperiod of time after the onset of the re-tightening.
 4. The method ofclaim 2, wherein steps (b) and (c) are repeated when the wheel brakeassembly (10) comes to a stop upon re-tightening.
 5. The method of claim2, wherein number of repetitions of steps (b) and (c) is limited.
 6. Themethod of claim 1 wherein said brief period of time during which thewheel brake assembly (10) is actuated in the release direction isdefined by a travel distance by which an actuating element of the wheelbrake assembly (10) is moved in the release direction.
 7. A method foractuating a mechanical system involving friction and having a springelasticity to increase a force exerted by the system beyond a forceattainable in a quasi-static state, the method comprising the steps of(a) actuating the system for a brief period of time in a releasedirection and then (b) tightened, the period of time of the actuation inthe release direction being selected to be so short that any reductionof the force exerted is imperceptible.
 8. The method of claim 7 furthercomprising repeating steps (a) and (b).
 9. The method of claim 8,wherein steps (a) and (b) are repeated after a predetermined period oftime after the onset of the re-tightening.
 10. The method of claim 8,wherein steps (a) and (b) are repeated when the system (10) comes to astop upon re-tightening.
 11. The method of claim 8, wherein number ofrepetitions of steps (a) and (b) is limited.
 12. The method of claim 7wherein said brief period of time during which the system (10) isactuated in the release direction is defined by a travel distance bywhich an actuating element of the system (10) is moved in the releasedirection.
 13. A method for actuating an electromechanical wheel brakeassembly having an electric motor, a brake actuator and means connectingthe electric motor to the brake actuator for converting rotary motion ofthe electric motor into a translational motion, the method comprisingthe steps of (a) initially actuating the electric motor in a tighteningdirection to cause the brake actuator to be pressed against a brake bodyto establish a quasi-static terminal braking state, then (b) actuatingthe electric motor for a brief period of time in a release directionopposite to the tightening direction, and then (c) again actuating theelectric motor in the tightening direction, said brief period of time ofthe actuation in the release direction being selected to be so shortthat any reduction of the braking force is imperceptible.
 14. The methodof claim 13 further comprising repeating steps (b) and (c).
 15. Themethod of claim 14, wherein steps (b) and (c) are repeated after apredetermined period of time after the onset of the re-tightening. 16.The method of claim 14, wherein steps (b) and (c) are repeated when thewheel brake assembly comes to a stop upon re-tightening.
 17. The methodof claim 14, wherein number of repetitions of steps (b) and (c) islimited.
 18. The method of claim 13 wherein said brief period of timeduring which the electric motor is actuated in the release direction isdefined by a travel distance by which the electric motor is moved in therelease direction.